This invention relates to a polyvinyl butyral-liquid crystal film-forming composition and to a film formed thereof. More particularly, the invention relates to a new and improved composition which when in the form of a film exhibits accentuated intensity and contrast of visible light waves reflected by the liquid crystals.
Cholesteric liquid crystalline phase materials, also referred to as cholesteric liquid crystals, their technology and their applications are reviewed in the book by Peter L. Carroll entitled "Cholesteric Liquid Crystals," June, 1973 (Ovum ltd., London). The materials, herein referred to for convenience at times simply as "liquid crystals," are a class of compounds that display a cholesteric mesophase or are in a cholesteric mesomorphic state within certain temperature limits. The cholesteric mesophase or mesomorphic state is a state of matter intermediate in molecular ordering between a crystalline solid and an isotropic liquid. In general, the liquid crystals are colorless in their solid and isotropic liquid states, assuming the coloration of their background or of light-absorptive materials added thereto. When the liquid crystals are in the cholesteric mesophase, and non-polarized white light is directed at them, the light is separated essentially into two components, one of which is transmitted and one of which is reflected or "scattered." The reflected light gives the material an iridescent color. The dominant wave length of the reflected light depends upon the liquid crystals and the angle of the incident light beam, and also is affected by stimuli applied to the liquid crystals, such as heat, physical stress, electricity, magnetism, and radiation, which are types of applied energy, the presence of other materials, which may be present as contaminants, and materials present in the environment and which act upon the liquid crystals.
An important property of the individual cholesteric liquid crystalline phase materials and of mixtures of different materials is that, all other conditions being equal, they always display the same color at a specific temperature. The characteristic color is changed by the application of one of the above-described stimuli, furnishing a direct, visible response which may be utilized in a detector system. A stimulus either can interact directly with the liquid crystals, or can be converted into a stimulus which does interact. The color responses to stimuli are utilized in systems for measuring and/or detecting specific temperatures and temperature changes, detecting radiation, measuring stress, monitoring the presence of gases and vapors, and other purposes. The iridescent colors create an aesthetic appeal which may be utilized in interior design, displays, novelty items, toys, and other applications.
The temperature sensitivity of the liquid crystals finds important use in thermometers and thermographic systems. In general, when viewed against a black absorptive background color, the liquid crystals change in color from red to green to blue to violet with increasing temperature in the mesophase temperature range, and the reverse color change takes place with decreasing temperature therein. Above and below the mesophase temperature range, the liquid crystals assume the color of the background.
Certain cholesteric liquid crystalline phase materials and their mixtures with other materials produce a metastable, ordered glass when fast-cooled from a first temperature in the mesophase temperature range thereof, to a second temperature below the glass-transition temperature (Tg) thereof, while retaining a color which is characteristic of the liquid crystal when at the first temperature, and when warmed from the second temperature to a temperature substantially equal to the glass-transition temperature, or to a temperature exceeding the latter, exhibit a color change which is irreversible by cooling from the glass-transition temperature, or the higher temperature (not reaching the first temperature), to the second temperature, thereby to visually indicate the occurrence of such warming. This irreversible color-temperature play, disclosed, for example, in U.S. Pat. No. 3,594,126, can be used to indicate that a temperature limit has been exceeded, such as in a frozen food thaw indicator.
Liquid crystals must be protected or shielded from the atmosphere unless they are used only for a relatively short period of time. Exposure to the atmosphere results in oxidation, causing the liquid crystals to deteriorate. Foreign particles from the atmosphere provide sites for crystallization and alter the delicately balanced mesomorphic state of the liquid crystals. Ultraviolet radiation may cause deterioration of the liquid crystals.
For protection against such exposure, protection against physical contacts, cleanliness, convenience in handling, and other reasons, liquid crystals in the past have been enclosed in various ways. In particular, liquid crystal-containing coatings, layers, films, sheets and the like have been provided, wherein the liquid crystals are protected by encapsulation or by dispersion in plastic materials. The liquid crystals have been encapsulated in gelatin-gum arabic, polyvinyl alcohol, zein, or other substances, such as disclosed in U.S. Pat. No. 3,585,381. Discrete naked aggregates of the liquid crystals have been dispersed in a substantially continuous solid polymeric matrix by drying an emulsion of the liquid crystals in an aqueous solution of a polymer such as polyvinyl alcohol or a polyacrylate, as disclosed in British Pat. No. 1,161,039, published Aug. 13, 1969, and U.S. Pat. No. 3,600,060, and by forming an organic solvent solution of a polymer and the liquid crystals, and removing the solvent to form a film or layer, suitable polymers including polyvinyl butyral, acrylic resin, styrene resins, polyester resins, epoxy resins, polyvinyl chloride, polyvinyl acetate, and polycarbonate, as disclosed in U.S. Pat. No. 3,620,889, and polyurethane, as disclosed in U.S. Pat. No. 3,872,050.
A two-component polymer system is required for making an acceptable liquid crystal-containing polyurethane film, and it is difficult to manufacture. Other films and sheets containing liquid crystals protected in the foregoing ways suffer from relatively low intensity of reflected or scattered colors and from poor color contrast. This has been the case with a liquid crystal-containing polyvinyl butyral film, in particular, which otherwise is a potentially useful film, owing to its ease of manufacture, and to the chemical resistance, water resistance and physical properties of the polymer.